Archaean mafic magmatism in the Kalgoorlie area of the Yilgarn Craton, Western Australia: a geochemical and Nd isotopic study of the petrogenetic andtectonic evolution of a greenstone belt

The compositional variation in the komatiites, basalts and gabbros of the K algoorlie greenstones illumines the magmatic-tectonic evolution of this Arc haean belt. Gabbro sills and basalts are the principal host for gold minera lization in the goldfields, yet have received relatively little attention. We present major. trace, rare earth element, and Nd isotope data for severa l suites of these rocks from around the Kalgoorlie gold mines. At the base of the Kalgoorlie sequence, the Hannan's Lake Serpentinite is an uncontamin ated komatiite that fractionated by olivine-pyroxene fractionation. The geo chemistry of the overlying high-Mg Devon Consols basalt was controlled by m oderate levels of crustal contamination and low-pressure fractionation of o livine and orthopyroxene. The Paringa Basalt is an internally variable high -Mg basalt and it is strongly contaminated by crustal material. Zircon xeno crysts in the mafic lavas show that they were erupted through continental c rust. AH of the mafic/ultramafic lavas appear to have been derived from a m antle plume; early uncontaminated magmas evolved through to strongly contam inated magmas, all derived from shallow parts of the plume (residual plagio clase). Later magmas were less contaminated and were derived from deeper in the plume (residual garnet). Layered gabbro sills intruded 20-30 Ma after the eruption of the (ultra)mafic volcanic sequence. The uncontaminated Gold en Mile-Aberdare gabbro is a highly fractionated tholeiitic rock. The Eurek a-Federal gabbro is a thin, weakly differentiated sill. The Williamstown pe ridotite is intruded deeper in the stratigraphic pile. Both of these latter sills may be contaminated. The transition from high-Mg to tholeiitic magma tism coincided closely with the onset of compressional regional deformation and felsic magmatism. The Eastern Goldfields lack classic features of subd uction - ophiolites, sheeted dykes, paired metamorphic belts, and regional granitoid zonation. Felsic volcanic rocks between 2710 and 2670 Ma do not s how an evolution in composition as would be expected in a transition from p lume to subduction tectonics. Regional compressive deformation may be relat ed to stress in the crust with the impingement of neighboring plumes, and/o r subsequent granitic plutonism. Crustal melting and generation of felsic m elts may have been a response to this massive mafic volcanic event. (C) 200 1 Elsevier Science B.V. All rights reserved.